A Manassas Park woman was killed yesterday morning when the automobile she was driving collided head-on with a vehicle driven by an off-duty Fairfax County police officer, authorities said.

Rebecca L. Marshall, 18, was driving north on Rte. 1 near Fort Belvoir, where she was a civilian employe, about 5:30 a.m. when a car heading south in the opposite lane swerved into her path, Fairfax County police spokesman Warren Carmichael said.

The driver of the southbound car was 24-year-old Beth A. Benham, a Fairfax County police officer at the Mount Vernon station, according to Carmichael.

Benham was on annual leave from the department at the time of the accident, officials said.

A police investigation of the accident indicated that Benham apparently had fallen asleep at the wheel, Carmichael said.

Marshall, of 1027 Evans St., died at 8:05 a.m. at Fairfax Hospital, where she was taken by helicopter after initial treatment at Mount Vernon Hospital.

Benham was in fair condition last night after surgery at Fairfax Hospital for multiple injuries, a hospital spokeswoman said.

A third driver, tentatively identified by officials as Jeffrey A. Cory of 4745 Hickory Nut Place in Dumfries, received minor injuries when his vehicle struck the two-car pile-up.

Because Benham is a police officer, Fairfax Commonwealth's Attorney Robert F. Horan Jr. will investigate the accident.

Horan said yesterday he had not had an opportunity to read a police report of the incident.

Benham apparently had been visiting friends and was on her way home when the crash took place, Carmichael said.

Neither Marshall nor Benham was wearing seat belts.